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Dis­cov­ery of what appears to be a new law of nature: more crowd­ing leads to fewer offspring

Why aren’t there more lions? That was what puz­zled McGill PhD stu­dent Ian Hat­ton, when he started look­ing at the pro­por­tion of preda­tors to prey across dozens of parks in East and South­ern Africa.

In this case, the answer had noth­ing to do with iso­lated human hunters. The parks were teem­ing with poten­tially tasty treats for the lions. So one might imag­ine that the pop­u­la­tion of lions in each park would increase to match the avail­able prey. Instead, what Hat­ton and the McGill-​led team dis­cov­ered was that, in a very sys­tem­atic way, in crowded set­tings, prey repro­duced less than they did in set­tings where their num­bers were smaller. More­over, they found this same pat­tern in a whole range of dif­fer­ent ecosys­tems. Their find­ings are pub­lished on 4 Sep­tem­ber in the jour­nal Science.

Lions hunt­ing a buf­falo, one of their prey species; Photo credit: McGill University/​Amaury Laporte

It’s a sur­pris­ing find­ing that sug­gests a level of orga­ni­za­tional struc­ture and func­tion in ecosys­tems that had not pre­vi­ously been rec­og­nized. Although biol­o­gists have long known of very reg­u­lar math­e­mat­i­cal laws gov­ern­ing func­tions in the body like metab­o­lism and growth, no study has ever shown that sim­i­lar kinds of laws may exist at a whole other level: that of ecosys­tems glob­ally. Some sci­en­tists are already sug­gest­ing that it may well be the dis­cov­ery of a new law of nature.

It came about by chance.

High school hol­i­days
“I went to high school in Zim­babwe and spent vaca­tions in the National parks there,” says Hat­ton, the lead author of the study that was just pub­lished in Sci­ence. “When I began my PhD in biol­ogy at McGill, I wanted to go back and com­pare whole com­mu­ni­ties of African ani­mals across pro­tected ecosys­tems to see how the num­bers of car­ni­vores are related to their her­bi­vore prey at the scale of whole land­scapes. So I gath­ered all the ani­mal cen­sus data I could for parks in east and south­ern Africa.”

When Hat­ton and his col­leagues then started putting it all together and crunch­ing the num­bers, sum­ming up all the car­ni­vores (lion, hyena, leop­ard, etc.) and her­bi­vores (buf­falo, zebra, impala, etc.) in these parks, they found a very unex­pected and reg­u­lar pat­tern. In every park they looked at, there seemed to be a very con­sis­tent rela­tion­ship of preda­tor to prey. But not in the sim­ple pat­tern they might have expected to find.

Some sur­pris­ing cal­cu­la­tions
“Until now, the assump­tion has been that when there is a lot more prey, you’d expect cor­re­spond­ingly more preda­tors,” says Hat­ton. “But as we looked at the num­bers, we dis­cov­ered instead, that in the lush­est ecosys­tems, no mat­ter where they are in the world, the ratio of preda­tors to their prey is greatly reduced. This is because with greater crowd­ing, prey species have fewer off­spring for every indi­vid­ual. In effect, the prey’s rates of repro­duc­tion are lim­ited, which lim­its the abun­dance of predators.”

Once they observed this pat­tern in one set­ting, the researchers then began ana­lyz­ing data about food pyra­mids, and the rela­tion­ship between preda­tors and prey in ecosys­tems as var­ied as the Indian Ocean, the Cana­dian Arc­tic and the trop­i­cal rain­forests. Over the course of the next few years they ana­lyzed data gath­ered about both plants and ani­mals from more than 1000 stud­ies done over the past 50 years cov­er­ing a range of grass­land, lake, for­est and ocean ecosys­tems around the world.

In all these dif­fer­ent set­tings, they found a sur­pris­ing con­sis­tency in the rela­tion of preda­tors to prey, and con­fir­ma­tion that rather than the num­bers of preda­tors increas­ing to match the avail­able prey, preda­tor pop­u­la­tions are lim­ited by the rate at which prey repro­duce. “We kept being aston­ished,” said Kevin McCann, of Guelph University’s Depart­ment of Inte­grated Biol­ogy, one of the study’s co-​authors. “This is just an amaz­ing pattern.”

A new law of nature?
What the researchers also found intrigu­ing was that the growth pat­terns they saw across whole ecosys­tems, where large num­bers of prey seemed nat­u­rally to repro­duce less, were very sim­i­lar to the pat­terns of growth in indi­vid­u­als. “Phys­i­ol­o­gists have long known that the speed of growth declines with size,” said co-​author Jonathan Davies from McGill’s Dept. of Biol­ogy. “The cells in an ele­phant grow more than 100 times more slowly than those of a mouse.”

“The dis­cov­ery of ecosystem-​level scal­ing laws is par­tic­u­larly excit­ing,” adds co-​author Michel Loreau, adjunct pro­fes­sor in McGill’s Biol­ogy Dept. and cur­rently at the Cen­tre national de recher­ché sci­en­tifique (CNRS) in France. “Their most intrigu­ing aspect is that they recur across lev­els of orga­ni­za­tion, from indi­vid­u­als to ecosys­tems, and yet ecosystem-​level scal­ing laws can­not be explained by their individual-​level coun­ter­parts. It seems that some basic processes re-​emerge across lev­els of orga­ni­za­tion, but we do not yet fully under­stand which ones and why.”

(Source: McGill Uni­ver­sity news release, 03.09.2015)